The present invention relates to a pneumatic tire in which run-flat performance can be improved while suppressing increase of tire weight.
Recent improvement of safety equipment for vehicles is striking. For example, ABS, an air bag system, a crushable body, and the like tend to become standard equipment even on a low-priced car and safety is now the most important challenge in a field of the automobile industry and this is not different in a field of a tire. Not only basic performance such as traveling, making a turn, and stopping but also traveling performance after a tire blowout is desired to be developed. In other words, the desire to a tire is growing, on which tire a driver can drive a car by himself/herself to the nearest gas station or automobile repair shop when the tire blowout has occurred in traveling such that the driver does not have to replace the tire at a road shoulder on an express way, at midnight, in bad weather, or the like.
The inventors studied a process of getting damaged of the tire after blowout and found that the process was generally as follows. If the tire blows and air leaks from the tire, vertical deflection of the tire increases, stress concentrates especially on a portion of a tire inner cavity, and a seriously distorted portion generates heat. Such heat generation causes early generation of separation and wear of rubber and causes a carcass that is a framework of the tire to be exposed. The carcass and a road surface or the carcass and the carcass rub against each other to cause rupture, the tire is fatally damaged, and traveling becomes impossible.
As a countermeasure against this, rigidity of the whole tire is normally increased so as to improve durability. Specifically, the number of plies of the carcass that is the framework of the tire is increased or a thickness of rubber is increased, for example. Because these means have been embodied substantially empirically, weight of the tire has been increased by a large amount.
The present invention has been accomplished with the above circumstances in view and it is an object of the present invention to provide a pneumatic tire in which run-flat performance as continuous traveling performance after blowout can be improved based on considering a model of stress analysis of a tire and improving a sectional shape of the tire to obtain an optimal sectional shape by which maximum stress applied to the tire can be reduced while minimizing increase in tire weight.
According to a first invention of the present invention, there is provided a pneumatic tire comprising a carcass extending from a tread portion through sidewall portions to bead cores in bead portions and satisfying the following expressions (1) to (4):
xe2x80x83Ra/Dxe2x89xa60.08xe2x80x83xe2x80x83(1);
Rb/Dxe2x89xa60.08xe2x80x83xe2x80x83(2);
0 less than xcfx86axe2x89xa650xc2x0xe2x80x83xe2x80x83(3); and
0 less than xcfx86bxe2x89xa650xc2x0xe2x80x83xe2x80x83(4),
where, in a tire meridional section in a standard state in which the tire is mounted on a standard wheel rim, inflated to a standard inner pressure, and not loaded, Ra is a radius of curvature of a first arc having a center Oa on a tire axial direction line X passing through a midpoint between a first point A and a second point B and on a tire inner cavity side and in contact with a tire center line at the first point A, the first point A being a point where a tire radial direction line Y passing through a wheel rim width position of the standard wheel rim intersects the tire center line passing through a midpoint of a thickness of the tire on a tread portion side and a second point B being a point where the tire radial direction line Y intersects the tire center line on a bead portion side, Rb is a radius of curvature of a second arc having a center Ob on the tire axial direction line X and on the tire inner cavity side and in contact with the tire center line at the second point B, xcfx86a is an angle formed by the tire axial direction line X and a straight line Oa-A connecting the center Oa and the first point A, xcfx86b is an angle formed by the tire axial direction line X and a straight line Ob-B connecting the center Ob and the second point B, and D is an outer diameter of the tire in the standard state.
In the pneumatic tire according to the first invention, it is preferable that the tire satisfies the above expressions (1) to (4) and that a tire constant T defined by the following expression (5) is 1.6xc3x9710xe2x88x923 or smaller:
T={(Ra/D)/Z}xc3x97{1xe2x88x92cos(xcfx86a/2)}+{(Rb/D)/Z}xc3x97{1xe2x88x92cos(xcfx86b/2)}xe2x80x83xe2x80x83(5),
where Z=h2/6, and
h=a thickness of each sidewall portion on the tire axial direction line X.
At this time, it is preferable that the thickness h of each the sidewall portion on the tire axial direction line X is 0.008 to 0.022 times the outer diameter D of the tire.
In the pneumatic tire according to the first invention, it is preferable that the tire satisfies the above expressions (1) to (4) and that a tire curvature constant V defined by the following expression (6) is 10xc3x9710xe2x88x923 or smaller:
V=(Ra/D)xc3x97{1xe2x88x92cos(xcfx86a/2)}+(Rb/D)xc3x97{1xe2x88x92cos(xcfx86b/2)}xe2x80x83xe2x80x83(6).
According to the second invention of the present invention, there is provided a pneumatic tire comprising a carcass extending from a tread portion through sidewall portions to bead cores in bead portions, wherein a tire section constant J per a unit circumferential length defined by the following expression (9) is 0.8 or smaller:                               J          =                                                    Ra                Z                            ⁢                              (                                  1                  -                                                            1                      -                                                                        H                          2                                                                          16                          ⁢                                                      xe2x80x83                                                    ⁢                                                      Ra                            2                                                                                                                                              )                                      +                                          Rb                Z                            ⁢                              (                                  1                  -                                                            1                      -                                                                        H                          2                                                                          16                          ⁢                                                      xe2x80x83                                                    ⁢                                                      Rb                            2                                                                                                                                              )                                                    ,                            (        9        )            
where, in a tire meridional section in a standard state in which the tire is mounted on a standard wheel rim, inflated to a standard inner pressure, and not loaded, Ra is a radius of curvature of a first arc having a center Oa on a tire axial direction line X passing through a midpoint between a first point A and a second point B and on a tire inner cavity side and in contact with a tire center line at the first point A, the first point A being a point where a tire radial direction line Y passing through a wheel rim width position of the standard wheel rim intersects the tire center line passing through a midpoint of a thickness of the tire on a tread portion side and a second point B being a point where the tire radial direction line Y intersects the tire center line on a bead portion side, Rb is a radius of curvature of a second arc having a center Ob on the tire axial direction line X and on the tire inner cavity side and in contact with the tire center line at the second point B, H is a distance in a radial direction of the tire from the first point A to the second point B, Z=h2/6, and h is a thickness of each the sidewall portion on the tire axial direction line X.
In the pneumatic tire according to the second invention, it is preferable that the thickness h of each the sidewall portion on the tire axial direction line X is 0.01 to 0.022 times an outer diameter D of the tire in the standard state and that an arc coefficient C of the tire defined by the following expression (10) is 5.0 or smaller:                     C        =                              Ra            ·                          (                              1                -                                                      1                    -                                                                  H                        2                                                                    16                        ⁢                                                  xe2x80x83                                                ⁢                                                  Ra                          2                                                                                                                                )                                +                      Rb            ·                          (                              1                -                                                      1                    -                                                                  H                        2                                                                    16                        ⁢                                                  xe2x80x83                                                ⁢                                                  Rb                          2                                                                                                                                )                                                          (        10        )            
In the pneumatic tire according to the second invention, the distance H in the radial direction of the tire is 0.085 times the outer diameter D of the tire in the standard state or smaller.